``We should see a significant reduction. That is what we are expecting in the population of the mosquitoes there,'' said Kevin Sullivan, a spokesman for the state Department of Public Health.

``We seem to have the situation under control,'' he said.

The outbreak has been traced to a swamp in Rhode Island, where state officials there last week confirmed the presence of the virus in mosquitoes. Rhode Island crews aerially sprayed the swamp to kill mosquitoes.

Gov. John G. Rowland announced late Thursday that the first laboratory tests on mosquitoes trapped in Connecticut, in Stonington and North Stonington, confirmed that some mosquitoes are carrying the virus, which although rare in humans, can be fatal.

In an update Friday, the agency said no new test results were expected before Monday. Trapping of mosquitoes was not expected to resume before Sunday. Spraying to kill mosquitoes might resume today, weather permitting, the health agency reported.

The sudden appearance of the virus has raised questions about where it came from.

John Anderson, director of the Connecticut Agricultural Experiment Station in New Haven, a state- sponsored research lab, said the virus is always present in birds, but usually at such low levels that few duck- biting mosquitoes pick it up. The virus develops in birds that live in freshwater swamps and is usually confined to these birds and mosquitoes that do not bite people.

In some years, when there is a wet spring, for example, the virus can become more prevalent among birds in old growth swamps. When many birds become infected, other mosquito species can pick up the virus and potentially transmit it to humans.

Whether through pesticide spraying or cold weather, the outbreak will not last much longer. Frost will kill adult mosquitoes. Also, mosquito eggs are not thought to be likely to carry the virus forward to another generation.

``Most of the laboratory studies indicate the virus does not infect ovarian tissue of the mosquito,'' said Theodore Andreadis, an entomologist at the expeiment station.

State Department of Environmental Protection crews have sprayed areas close to the Rhode Island border most likely to harbor infected mosquitoes. In that spraying, the agency used resmethrin, an insecticide that kills mosquitoes within 30 minutes of application.

It was chosen because it is the least toxic product available, the agency said, and breaks down in sunlight within four hours. It does not kill bees and is not harmful to wildlife or people. It can, however, be toxic to fish, and the state has avoided direct application to lakes or streams.

Spraying so far has been done on the ground. Connecticut does not allow aerial spraying unless an emergency is declared by health or environmental officials. Rhode Island officials chose to aerially spray because they said there was no other way to reach interior parts of the swamp.

``At this point we are not recommending aerial spraying,'' Sullivan said.

The agency believes any infected mosquitoes are confined to the area adjacent to the Westerly, R.I., border, Sullivan said. The agency said that people in Stonington or North Stonington with questions should contact local officials or the Department of Public Health at (860) 509-7994 Monday through Friday from 8:30 a.m. until 4:30 p.m.